In single-crystalline metals, the sources of size effects depend on the
sample length scale. In bulk samples, the interaction of dislocations
with each other is responsible for size effects which is commonly termed
forest hardening. The Taylor-like hardening models are usually
incorporated to capture the forest hardening which states that the
strength increases as the dislocation density increases. As an example,
the fact that the nanoindentation hardness increases as the indentation
depth decreases is justified as an increase in the density of
geometrically necessary dislocations. In the cases of small length
scales, several experiments on whiskers, wires, and micropillars have
demonstrated that the sources of size effects are different from those
of bulk material. In the case of nanoindentation of nanoscale samples,
it has been experimentally shown that the hardness decreases as the
density of geometrically necessary dislocations increases in the region
of small indentation depths. It shows that the size effects theory of
bulk material cannot be extended to the indentation of nanoscale
samples. The present work incorporates the large scale atomistic
simulation to investigate the size effects in a nanoscale single crystal
Ni thin film during indentation. The results show that the hardness
decreases as the dislocation density increases, and the forest hardening
model cannot capture the strength size effects during nanoindentation at
small length scales. It is observed that the size effects are initially
controlled by dislocation nucleation and source exhaustion. As the
indentation depth increases, the dislocation length and density
increase. Consequently, the number of dislocation sources and their
characteristic length increase which decreases the material strength.
Finally, increasing the dislocation length and density, the dislocation
interaction mechanism also becomes important. (C) 2015 Elsevier B.V. All
rights reserved.